Material for electrode of power storage device, power storage device, and electrical appliance

ABSTRACT

To improve the reliability of a power storage device. A granular active material including carbon is used, and a net-like structure is formed on part of a surface of the granular active material. In the net-like structure, a carbon atom included in the granular active material is bonded to a silicon atom or a metal atom through an oxygen atom. Formation of the net-like structure suppresses reductive decomposition of an electrolyte solution, leading to a reduction in irreversible capacity. A power storage device using the above active material has high cycle performance and high reliability.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to a material for an electrode of a powerstorage device, a power storage device, and an electrical appliance.

2. Description of the Related Art

In recent years, a variety of power storage devices, for example,non-aqueous secondary batteries such as lithium ion batteries (LIBs),lithium ion capacitors (LICs), and air cells have been activelydeveloped. In particular, demand for lithium ion batteries with highoutput and high energy density has rapidly grown with the development ofthe semiconductor industry, for example, portable information terminalssuch as mobile phones, smartphones, and laptop computers, portable musicplayers, and digital cameras; medical equipment; and next-generationclean energy vehicles such as hybrid electric vehicles (REVS), electricvehicles (EVs), and plug-in hybrid electric vehicles (PHEVs). Thelithium ion batteries are essential for today's information society asrechargeable energy supply sources.

A negative electrode of the power storage devices such as the lithiumion batteries and the lithium ion capacitors includes at least anegative electrode current collector and a negative electrode activematerial layer provided on a surface of the negative electrode currentcollector. The negative electrode active material layer contains anegative electrode active material such as carbon or silicon, which canstore and release lithium ions serving as carrier ions.

A negative electrode of a lithium ion battery using a graphite-basedcarbon material is formed by mixing graphite (black lead) that is anegative electrode active material, acetylene black (AB) as a conductiveadditive, and polyvinylidene fluoride (PVDF) that is a resin as a binderto form slurry, applying the slurry over a negative electrode currentcollector, and drying the slurry, for example.

A lithium ion battery or a lithium ion capacitor has a problem in thatirreversible capacity caused by repetitive insertion/extraction oflithium ions into/from the negative electrode active material isgenerated.

A negative electrode of a lithium ion battery or a lithium ion capacitorhas an extremely low electrode potential and a high reducing ability.Accordingly, an electrolyte solution containing an organic solvent isreduced and decomposed, and decomposed matters form a film on a surfaceof the negative electrode. The formation of the film generatesirreversible capacity, so that part of discharge capacity is lost.

As a technique for reducing the loss of discharge capacity, for example,a technique in which a surface of a negative electrode active materialis covered with a metal oxide film, a silicon oxide film, or the likehas been known (e.g., Patent Document 1). The formation of the aboveoxide film on the surface of the negative electrode active material cansuppress formation of the film formed on the surface of the negativeelectrode due to the decomposition, and thus can reduce irreversiblecapacity.

REFERENCE Patent Document

-   [Patent Document 1] Japanese Published Patent Application No.    H11-329435

SUMMARY OF THE INVENTION

However, capacity loss cannot be reduced sufficiently in conventionalpower storage devices.

For example, in the case of using a carbon-based material as a negativeelectrode active material, the cause of generation of irreversiblecapacity is not only the film formed on the surface of the negativeelectrode due to the decomposition, but can be, for example, afunctional group or a dangling bond at an end of the negative electrodeactive material. The functional group or the dangling bond is unstableand a structure of the carbon-based material is readily changed, so thatirreversible capacity is likely to be formed.

A functional group or a dangling bond exists even when a surface of anegative electrode active material is covered with an oxide film as in,for example, Patent Document 1. Therefore, the capacity loss cannot bereduced sufficiently only by the conventional method of covering asurface of a negative electrode active material with an oxide film.

The above problems exist not only in lithium ion batteries but also inlithium ion capacitors.

An object of one embodiment of the present invention is to reduceirreversible capacity of a power storage device.

An object of one embodiment of the present invention is to reduce thenumber of functional groups or dangling bonds at an end of a materialserving as an active material.

Another object of one embodiment of the present invention is to improvethe reliability of a power storage device.

In one embodiment of the present invention, a granular active materialincluding carbon is used, and a net-like structure is formed on part ofa surface of the granular active material. The net-like structure isformed by a plurality of bonds between a carbon atom included in thegranular active material and a silicon atom or a metal atom through anoxygen atom. Formation of the net-like structure suppresses reductivedecomposition of an electrolyte solution, leading to a reduction inirreversible capacity. Furthermore, the number of functional groups ordangling bonds existing on the surface of the granular active materialis reduced in order to reduce irreversible capacity.

One embodiment of the present invention is a material for an electrodeof a power storage device, which includes a granular active material andhas a net-like structure on part of a surface of the granular activematerial. The net-like structure is formed by a plurality of bondsbetween a carbon atom included in the granular active material and asilicon atom or a metal atom through an oxygen atom.

In the above embodiment of the present invention, for example, in thecase where the granular active material is a graphite particle includinga plurality of graphene layers, a net-like structure may be providedacross ends of the plurality of graphene layers on part of the surfaceof the granular active material. Thus, a change in structure, such asseparation of graphene layers by insertion/extraction of carrier ionsinto/from graphite particles, is suppressed.

In the above embodiment of the present invention, n (n is a naturalnumber) oxide layers each including a bond of the silicon atom or themetal atom and the oxygen atom may be provided over the net-likestructure.

Another embodiment of the present invention is a power storage device inwhich a negative electrode active material layer of a negative electrodeincludes the above material for an electrode of a power storage device.

Another embodiment of the present invention is an electrical applianceincluding the above power storage device.

In one embodiment of the present invention, irreversible capacity can bereduced, and thus, loss of discharge capacity can be reduced.Furthermore, the number of functional groups or dangling bonds at an endof a material serving as an active material can be reduced. Moreover,the reliability of a power storage device can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B illustrate an example of a material for an electrode ofa power storage device.

FIGS. 2A and 2B illustrate an example of a material for an electrode ofa power storage device.

FIG. 3 illustrates an example of a material for an electrode of a powerstorage device.

FIGS. 4A and 4B are each a flow chart of a method for producing amaterial for an electrode of a power storage device.

FIGS. 5A to 5D illustrate an example of a negative electrode of a powerstorage device.

FIGS. 6A to 6C illustrate an example of a negative electrode of a powerstorage device.

FIGS. 7A and 7B illustrate an example of a power storage device.

FIG. 8 illustrates an example of a power storage device.

FIGS. 9A and 9B illustrate an example of a power storage device.

FIG. 10 illustrates examples of electrical appliances.

FIGS. 11A to 11C illustrate examples of electrical appliances.

FIGS. 12A and 12B illustrate an example of an electrical appliance.

FIG. 13 is a graph showing results of CV measurement.

FIG. 14 is an image observed with SEM.

FIGS. 15A and 15B show images observed with STEM and results of EDX.

FIGS. 16A and 16B are graphs showing results of CV measurement.

FIG. 17A is a graph showing results of CV measurement, and FIG. 17B is agraph showing the capacity of decomposition of an electrolyte solution.

FIG. 18 is a graph showing cycle performance.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below. Note thatit will be readily appreciated by those skilled in the art that contentsof the embodiments can be modified without departing from the spirit andscope of the present invention. Thus, the present invention should notbe limited to, for example, the description of the followingembodiments.

Note that the contents in different embodiments can be combined with oneanother as appropriate. In addition, the contents in differentembodiments can be replaced with one another as appropriate.

The ordinal numbers such as “first” and “second” are used to avoidconfusion between components and do not limit the number of eachcomponent.

Embodiment 1

In this embodiment, an example of a material for an electrode of a powerstorage device will be described.

<Structural Example of Electrode Material of Power Storage Device>

First, a structural example of a material for an electrode of a powerstorage device of this embodiment will be described with reference toFIGS. 1A and 1B, FIGS. 2A and 2B, and FIG. 3.

The material for an electrode of a power storage device includes agranular active material 110 as illustrated in FIG. 1A.

The shape of the granular active material 110 is not particularlylimited. The granular active material 110 may have a spherical shape(powdered state), a plate-like shape, an angular shape, a column shape,a needle-like shape, or a scale-like shape, for example. Note that afilm-like active material may be used instead of the granular activematerial 110.

As a material of the granular active material 110, a carbon-basedmaterial (e.g., graphite) can be used.

Graphite is a layered compound in which a plurality of graphene layersis stacked in parallel to each other by van der Waals forces. Thegraphene layer is a sheet composed of a hexagonal net pattern of aone-atom thick layer of carbon formed by carbon atoms which arecovalently bonded to each other to form sp² hybrid orbitals andtricoordinate with each other at an angle of 120° in a surface. Notethat the graphene layer may partly include defects or functional groups.

Examples of graphite include low crystalline carbon such as soft carbonand hard carbon and high crystalline carbon such as natural graphite,kish graphite, pyrolytic graphite, mesophase pitch based carbon fiber,meso-carbon microbeads, mesophase pitches, petroleum-based or coal-basedcoke, and the like.

The particle diameter of the granular active material 110 is notparticularly limited, and may be greater than or equal to 6 μm and lessthan or equal to 30 μm, for example.

A net-like structure of C—O-M bonds illustrated in FIG. 1B is formed onpart of a surface of the granular active material 110. The C—O-M bond isa bond where a plurality of carbon atoms included in the granular activematerial 110 is bonded to a silicon atom or a metal atom through anoxygen atom. C represents a carbon atom included in the granular activematerial 110, O represents an oxygen atom, and M represents, forexample, a niobium atom, a titanium atom, a vanadium atom, a tantalumatom, a tungsten atom, a zirconium atom, a molybdenum atom, a hafniumatom, a chromium atom, an aluminum atom, or a silicon atom.

It is preferable that the net-like structure do not have electronconductivity. Furthermore, it is preferable that the net-like structurehave a function of allowing passage of a carrier ion of a power storagedevice. Note that examples of the carrier ion include a lithium ion thatis used for a lithium ion battery or a lithium ion capacitor; an alkalimetal ion (e.g., a sodium ion or a potassium ion); an alkaline earthmetal ion (e.g., a calcium ion, a strontium ion, or a barium ion, aberyllium ion, or a magnesium ion).

The net-like structure is preferably formed not on the entire surface ofthe granular active material 110 but on part of the surface of thegranular active material 110. In the case where a plurality of granularactive materials 110 is in contact with each other as illustrated inFIG. 1A, the net-like structure is preferably formed in a region otherthan the contact portions. The net-like structure covers not the entiresurface of the granular active material 110, so that cell reaction ispossible and the decomposition reaction of an electrolyte solution canbe suppressed.

The net-like structure illustrated in FIG. 1B is referred to as achemical network structured interface (CNSI) layer.

The CNSI layer is a three-dimensional net-like structure that is formedby chemical bonds of carbon included in the granular active material andan oxide of metal, silicon, or the like.

The CNSI layer can make a surface of the granular active material stableas compared with a film formed on an electrode surface due todecomposition of an electrolyte solution. Thus, the CNSI layer functionsas a protection layer. The CNSI layer is dense and has good adhesion tothe granular active material. For this reason, providing the CNSI layercan reduce an area of the granular active material in direct contactwith an electrolyte solution, suppress the decomposition of theelectrolyte solution in a power storage device, and reduce irreversiblecapacity that causes a decrease in the initial capacity of the powerstorage device.

Note that n (n is a natural number) oxide layers each including a M-Obond formed of M and O may be provided over the net-like structure. Insuch a case, O in the oxide layer that is adjacent to the CNSI layerbonds to M in the CNSI layer. Since the same oxide is used for the CNSIlayer and the oxide layer, a connection between the CNSI layer and theoxide layer is stable.

The netlike structure can be formed, for example, in such a manner thata coating film 111 (a film formed of an oxide including O and M) isformed on part of the surface of the granular active material 110 asillustrated in FIG. 1A, and carbon included in the granular activematerial 110 is bonded to an oxide included in the coating film 111. Inthat case, an oxygen atom included in the coating film 111 isrepresented by O, and a metal atom or a silicon atom included in thecoating film 111 is represented by M.

The coating film of one embodiment of the present invention is anartificial film provided in advance before a power storage device ischarged and discharged, and is clearly distinguished from a film formeddue to the decomposition reaction between an electrolytic solution andan active material in this specification and the like.

A structural example of the material for an electrode of a power storagedevice, which includes the CNSI layer, will be described with referenceto FIGS. 2A and 2B. Here, description is made on a case of forming thenetlike structure in such a manner that a coating film (a film formed ofan oxide including a metal atom or a silicon atom) is formed on graphiteparticles, and carbon included in the graphite particles is bonded to anoxide included in the coating film. FIG. 2A is a schematic view ofgraphite particles used for the netlike structure. FIG. 2B is aschematic view illustrating the case of forming the netlike structureusing the graphite particles in FIG. 2A. Note that in FIGS. 2A and 2B, arelatively small black sphere represents a carbon (C) atom, a relativelylarge black sphere represents a metal or silicon (M) atom, a gray sphererepresents an oxygen (O) atom, and a white sphere represents a hydrogen(H) atom. Note that for convenience, the size of the spheres may bedifferent from the actual size of the atoms.

As illustrated in FIG. 2A, a graphite particle 211 is composed of aplurality of graphene layers, and a dangling bond or a functional groupsuch as an OH group or a COOH group exists at an end of the graphenelayer. At this time, an end of the graphite particle 211 is unstable.Accordingly, when the graphite particle 211 is used for a power storagedevice, decomposition of an electrolyte solution and separation ofgraphene layers due to insertion and extraction of carrier ions arelikely to occur.

When a coating film (a film of an oxide) is formed over the graphiteparticle 211 illustrated in FIG. 2A and carbon included in the graphiteparticle is bonded to an oxide included in the coating film, danglingbonds or functional groups at ends of the plurality of graphene layersreact with oxides as illustrated in FIG. 2B.

At this time, bond portions of oxides and carbon atoms at the ends ofthe plurality of graphene layers on part of a surface of the graphiteparticle 211 (on part of the surface of the granular active material110) correspond to a net-like structure 212 formed of C—O-M bonds. Inother words, the net-like structure 212 is three-dimensionally formedacross ends of the plurality of graphene layers on the surface of thegranular active material 110. With the net-like structure 212 formedacross ends of the plurality of graphene layers, separations of graphenelayers due to insertion and extraction of carrier ions can be prevented;therefore, a change in the structure of the graphite particle 211 can beprevented.

Moreover, n oxide layers 213 are formed over the net-like structure 212.Note that the oxide layer 213 may be extended to the surface of thegraphite particle 211 positioned in a portion other than the ends of theplurality of graphene layers.

FIG. 3 is a schematic diagram illustrating the case where the electrodematerial illustrated in FIG. 2B is in contact with an electrolytesolution. As illustrated in FIG. 3, when the three-dimensional net-likestructure 212 formed of C—O-M bonds and the oxide layer 213 are providedbetween the graphite particle 211 and an electrolyte solution 214including a molecule containing lithium, an area of the graphiteparticle 211 in direct contact with the electrolyte solution 214 isreduced, so that decomposition of the electrolyte solution 214 issuppressed. Moreover, a film due to the decomposition of the electrolytesolution is less likely to be formed on the surface of the graphiteparticle 211 including the net-like structure 212.

As described above with reference to FIGS. 1A and 1B, FIGS. 2A and 2B,and FIG. 3, in the material for an electrode of a power storage of thisembodiment, the net-like structure formed of C—O-M bonds is provided onpart of the surface of the granular active material, so that the surfaceof the granular active material is stabilized. Thus, decomposition ofthe electrolyte solution can be suppressed.

<Method of Producing Electrode Material of Power Storage Device>

Next, as an example of a method for producing the material for anelectrode of a power storage device that includes the netlike structure,a method for producing the material for an electrode of a power storagedevice illustrated in FIG. 1A will be described with reference to FIGS.4A and 4B. Here, a producing method using a sol-gel method and aproducing method using a polysilazane method will be described asexamples.

[Method of Producing Electrode Material of Power Storage Device UsingSol-Gel Method]

At Step S150 in FIG. 4A, metal alkoxide or silicon alkoxide and astabilizing agent are added to a solvent, and the mixture is stirred toform a solution.

As the solvent, toluene can be used, for example.

As the stabilizing agent, ethyl acetoacetate can be used, for example.

When a silicon oxide film is formed as the coating film 111, forexample, silicon ethoxide, methoxide, or the like can be used asalkoxide.

Next, at Step S151, the granular active material 110 is added to thesolution, and the mixture is stirred. Thus, the solution becomes a thickpaste and the surface of the granular active material 110 is coveredwith the alkoxide.

At Step S152, the alkoxide on the surface of the granular activematerial 110 is turned into a gel by a sol-gel method.

At Step S152, a small amount of water is added to the solution includingthe granular active material 110 so that the alkoxide reacts with water,whereby a sol-state decomposition product is formed. Here, the term “asol state” refers to a state where solid fine particles aresubstantially uniformly dispersed in a liquid. Note that the smallamount of water may be added to the solution including the activematerial by exposing the solution to the air. For example, in the casewhere silicon ethoxide (Si(OEt)₄) is used as the alkoxide, hydrolysisreaction is expressed by Formula 1.Si(OEt)₄+4H₂O→Si(OEt)_(4-x)(OH)_(x) +xEtOH (x is an integer of 4 orless)  (Formula 1)

Next, the sol-state decomposition product is dehydrated and condensed tobe a reactant which is a gel. Here, “being a gel” refers to being in astate where a three-dimensional network structure is developed due toattractive interaction between solid fine particles and thedecomposition product is solidified. In the case where silicon ethoxide(Si(OEt)₄) is used as the alkoxide, the condensation reaction isexpressed by Formula 2.2Si(OEt)_(4-x)(OH)_(x)→(OEt)_(4-x)(OH)_(x-1)Si—O—Si(OH)_(x-1)(OEt)_(4-x)+H₂O(x is an integer of 4 or less)  (Formula 2)

When silicon ethoxide is used as the alkoxide and a graphite particle isused as the granular active material 110, condensation reaction ofhydrate of silicon ethoxide occurs, whereby the net-like structureformed of C—O-M bonds is formed at an end of the granular activematerial 110. For example, when carbon in the granular active material110 is represented by C, a functional group is represented by OH orCOOH, and the granular active material 110 including the functionalgroup is represented by C—OH or C—COOH, condensation reaction isexpressed by Formula 3 or Formula 4.Si(OEt)_(4-x)(OH)_(x)+C—OH→C—O—Si(OEt)_(4-x)(OH)_(x-1)+H₂O (x is aninteger of 4 or less)  (Formula 3)Si(OEt)_(4-x)(OH)_(x)+C—COOH→C—CO—O—Si(OEt)_(4-x)(OH)_(x-1)+H₂O (x is aninteger of 4 or less)  (Formula 4)

The condensation reaction is determined by the kind of hydrate ofsilicon ethoxide and the number of functional groups at ends of thegranular active material 110. Note that in the case where an end of thegranular active material 110 has a dangling bond, carbon having adangling bond bonds to the above oxide; thus, a C—O-M bond is formed.

Through these steps, the net-like structure formed of C—O-M bonds can beformed on part of the surface of the granular active material 110.

After that, heat treatment is performed under atmospheric pressure atStep S153, whereby the material for an electrode of a power storagedevice can be produced. The temperature of the heat treatment is higherthan or equal to 300° C. and lower than or equal to 900° C., preferablyhigher than or equal to 500° C. and lower than or equal to 800° C.

By the producing method using a sol-gel method shown in FIG. 4A, theelectrode material in which the net-like structure formed of C—O-M bondsis formed on part of the surface of the granular active material can beproduced.

[Method of Producing Electrode Material of Power Storage Device UsingPolysilazane Method]

At Step S160, a stabilizing agent is added to a solvent, and the mixtureis stirred to form a solution.

As the solvent, toluene can be used, for example.

As the stabilizing agent, ethyl acetoacetate can be used, for example.

In the case where, for example, a silicon oxide film is formed as thecoating film 111, at Step S161, the granular active material 110 and apolysilazane-containing solution which contains perhydropolysilazane areadded to the above solution, and the mixture is stirred. Thus, thesolution becomes a thick paste.

Next, at Step S162, the sample is kept in the air, and heat treatment isperformed at Step S163 so that inversion of perhydropolysilazane isperformed. Note that the heat treatment temperature is higher than orequal to 30° C. and lower than or equal to 600° C., preferably higherthan or equal to 100° C. and lower than or equal to 200° C. The heattreatment is not necessarily performed. For example, the inversion ofperhydropolysilazane can be performed by keeping the sample attemperature higher than or equal to 15° C. and lower than 30° C. for acertain period of time. When perhydropolysilazane is SiH₂NH, theinversion reaction is expressed by Formula 5.SiH₂NH+2H₂O→SiO₂+NH₃+2H₂  (Formula 5)

At this time, a compound including Si(OH) is generated as a by-product.This compound reacts with a functional group at the end of the granularactive material 110, whereby the net-like structure formed of C—O-Mbonds is formed. For example, when carbon in the granular activematerial 110 is represented by C, a functional group is represented byOH or COOH, and the granular active material 110 including thefunctional group is represented by C—OH or C—COOH, the generationreaction is expressed by Formula 6 and Formula 7.Si(OH)+C—OH→C—O—Si—+H₂O  (Formula 6)Si(OH)+C—COOH→C—CO—O—Si—+H₂O  (Formula 7)

Through these steps, the net-like structure formed of C—O-M bonds can beformed on part of the surface of the granular active material 110.

By the producing method using a polysilazane method shown in FIG. 4B,the electrode material in which the net-like structure formed of C—O-Mbonds is formed on part of the surface of the granular active materialcan be produced.

Embodiment 2

In this embodiment, a negative electrode of a power storage device usingthe material for an electrode of a power storage device described inEmbodiment 1 and a method for forming the negative electrode will bedescribed with reference to FIGS. 5A to 5D.

As illustrated in FIG. 5A, a negative electrode 200 includes a negativeelectrode current collector 201 and a negative electrode active materiallayer 202 provided on one or both surfaces (on the both surfaces in thedrawing) of the negative electrode current collector 201.

The negative electrode current collector 201 is formed using a highlyconductive material which is not alloyed with a carrier ion such aslithium. For example, stainless steel, copper, nickel, or titanium canbe used. In addition, the negative electrode current collector 201 canhave a foil-like shape, a plate-like shape (sheet-like shape), anet-like shape, a punching-metal shape, an expanded-metal shape, or thelike as appropriate. The negative electrode current collector 201preferably has a thickness of more than or equal to 10 μm and less thanor equal to 30 μm.

The negative electrode active material layer 202 is provided on one orboth surfaces of the negative electrode current collector 201. For thenegative electrode active material layer 202, the electrode materialdescribed in Embodiment 1 can be used.

In this embodiment, the negative electrode active material layer 202formed by mixing and baking the electrode material described inEmbodiment 1, a conductive additive, and a binder is used.

The negative electrode active material layer 202 is described withreference to FIG. 5B. FIG. 5B is a cross-sectional view of part of thenegative electrode active material layer 202. The negative electrodeactive material layer 202 includes the electrode material described inEmbodiment 1, a conductive additive 204, and a binder (not illustrated).

The conductive additive 204 has a function of increasing theconductivity between the granular negative electrode active materials203 or between the granular negative electrode active material 203 andthe negative electrode current collector 201, and is preferably added tothe negative electrode active material layer 202, for example, Amaterial with a large specific surface is desirably used as theconductive additive 204, and acetylene black (AB) or the like can beused. Alternatively, a carbon material such as a carbon nanotube,graphene, or fullerene can be used as the conductive additive 204. Notethat the case of using graphene is described later as an example.

As the hinder, a material which at least binds the negative electrodeactive material, the conductive additive, and the current collector isused. Examples of the binder include resin materials such aspolyvinylidene fluoride (PVDF), a vinylidenefluoride-hexafluoropropylene copolymer, a vinylidenefluoride-tetrafluoroethylene copolymer, styrene-butadiene copolymerrubber, polytetrafluoroethylene, polypropylene, polyethylene, andpolyimide.

The negative electrode 200 is formed in the following manner. First, theelectrode material made by the method described in Embodiment 1 is mixedto a solvent such as NMP (N-methylpyrrolidone) in which a vinylidenefluoride based polymer such as polyvinylidene fluoride is dissolved,whereby slurry is formed.

Next, the slurry is applied on one or both surfaces of the negativeelectrode current collector 201, and dried. In the case where theapplication step is performed on both surfaces of the negative electrodecurrent collector 201, the negative electrode active material layers 202are formed on the surfaces at the same time or one by one. Then, rollingwith a roller press machine is performed, whereby the negative electrode200 is formed.

Next, an example of using graphene as the conductive additive added tothe negative electrode active material layer 202 is described withreference to FIGS. 5C and 5D.

FIG. 5C is a plan view of part of the negative electrode active materiallayer 202 using graphene. The negative electrode active material layer202 includes the granular negative electrode active materials 203 whichcorrespond to the granular active materials 110 described in Embodiment1 and graphenes 205. The graphene 205 covers a plurality of granularnegative electrode active materials 203 and at least partly surround theplurality of granular negative electrode active materials 203. A binderwhich is not illustrated may be added; however, in the case where thegraphenes 205 are contained so that they are bound to each other tofunction well as a binder, the binder is not necessarily added. In theplan view of the negative electrode active material layer 202, differentgraphenes 205 cover the surfaces of the granular negative electrodeactive materials 203. The granular negative electrode active materials203 may be partly exposed.

FIG. 5D is a cross-sectional view of part of the negative electrodeactive material layer 202 in FIG. 5C. FIG. 5D illustrates the granularnegative electrode active materials 203 and the graphenes 205. In theplan view of the negative electrode active material layer 202, thegraphene 205 covers a plurality of granular negative electrode activematerials 203. The graphene 205 has a linear shape when observed in thecross-sectional view. The plurality of granular negative electrodeactive materials 203 are at least partly surrounded with one graphene205 or a plurality of graphenes 205 or sandwiched between the pluralityof graphenes 205. Note that the graphene 205 has a bag-like shape andthe plurality of granular negative electrode active materials 203 issurrounded by the graphene 205 in some cases. In addition, the granularnegative electrode active materials 203 are partly not covered with thegraphene 205 and exposed in some cases.

The thickness of the negative electrode active material layer 202 ispreferably selected as appropriate in the range of 20 μm to 150 μm.

Note that the negative electrode active material layer 202 may bepredoped with lithium. Predoping with lithium may be performed in such amanner that a lithium layer is formed on a surface of the negativeelectrode active material layer 202 by a sputtering method.Alternatively, lithium foil is provided on the surface of the negativeelectrode active material layer 202, whereby the negative electrodeactive material layer 202 can be predoped with lithium.

As an example of the granular negative electrode active material 203,there is a material whose volume is expanded by occlusion of carrierions. Thus, the negative electrode active material layer including sucha material gets friable and is partly broken by charge and discharge,which reduces the reliability (e.g., cycle performance) of the powerstorage device.

However, even when the volume of the negative electrode active materialis expanded due to charge and discharge, the graphene 205 partly coversthe periphery of the granular negative electrode active materials 203,which allows prevention of dispersion of the negative electrode activematerials and the breakdown of the negative electrode active materiallayer. That is to say, the graphene 205 has a function of maintainingthe bond between the positive electrode active materials even when thevolume of the positive electrode active material fluctuates by chargeand discharge. For this reason, a binder does not need to be used informing the negative electrode active material layer 202. Accordingly,the proportion of the negative electrode active material in the negativeelectrode active material layer 202 with certain weight (certain volume)can be increased, leading to an increase in charge and dischargecapacity per unit weight (unit volume) of the electrode.

The graphene 205 has conductivity and is in contact with a plurality ofgranular negative electrode active materials 203; thus, it also servesas a conductive additive. That is, a conductive additive does not needto be used in forming the negative electrode active material layer 202.Accordingly, the proportion of the negative electrode active material inthe negative electrode active material layer 202 with certain weight(certain volume) can be increased, leading to an increase in charge anddischarge capacity per unit weight (unit volume) of the electrode.

Furthermore, the graphene 205 efficiently forms a sufficient conductivepath of electrons in the negative electrode active material layer 202,which increases the conductivity of the negative electrode 200.

Note that the graphene 205 also functions as a negative electrode activematerial that can occlude and release carrier ions, leading to anincrease in charge capacity of the negative electrode 200.

Next, a method for forming the negative electrode active material layer202 in FIGS. 5C and 5D is described.

First, the electrode material described in Embodiment 1 and a dispersionliquid containing graphene oxide are mixed to form slurry.

Next, the slurry is applied onto the negative electrode currentcollector 201. Next, drying is performed in a vacuum for a certainperiod of time to remove a solvent from the slurry applied onto thenegative electrode current collector 201. Then, rolling with a rollerpress machine is performed.

Then, the graphene oxide is electrochemically reduced with electricenergy or thermally reduced by heating treatment to form the graphene205. Particularly in the case where electrochemical reduction treatmentis performed, a proportion of C(π)—C(π) double bonds of graphene formedby the electrochemical reduction treatment is higher than that ofgraphene formed by heating treatment; therefore, the graphene 205 havinghigh conductivity can be formed. Through the above steps, the negativeelectrode active material layer 202 used as a conductive additive can beformed on one or both surfaces of the negative electrode currentcollector 201, and thus the negative electrode 200 can be formed.

Embodiment 3

In this embodiment, a structure of a lithium ion battery as a powerstorage device and a method for manufacturing the lithium ion batteryare described.

(Positive Electrode)

First, a positive electrode and a method for forming the positiveelectrode will be described.

FIG. 6A is a cross-sectional view of a positive electrode 250. In thepositive electrode 250, a positive electrode active material layer 252is formed over a positive electrode current collector 251.

For the positive electrode current collector 251, a highly conductivematerial such as a metal typified by stainless steel, gold, platinum,zinc, iron, aluminum, or titanium, or an alloy of these metals can beused. Note that the positive electrode current collector 251 can beformed using an aluminum alloy to which an element which improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added. Further alternatively, the positive electrodecurrent collector 251 may be formed using a metal element which formssilicide by reacting with silicon. Examples of the metal element whichforms silicide by reacting with silicon include zirconium, titanium,hafnium, vanadium, niobium, tantalum, chromium, molybdenum, tungsten,cobalt, nickel, and the like. The positive electrode current collector251 can have a foil-like shape, a plate-like shape (a sheet-like shape),a net-like shape, a punching-metal shape, an expanded-metal shape, orthe like as appropriate.

In addition to a positive electrode active material, a conductiveadditive and a binder may be included in the positive electrode activematerial layer 252.

As the positive electrode active material of the positive electrodeactive material layer 252, a compound such as LiFeO₂, LiCoO₂, LiNiO₂,LiMn₂O₄, V₂O₅, Cr₂O₅, or MnO₂ can be used.

Alternatively, an olivine-type lithium-containing composite phosphate(LiMPO₄ (general formula) (M is one or more of Fe(II), Mn(II), Co(l),and Ni(II))) can be used for the positive electrode active material.Typical examples of the general formula LiMPO₄ include LiFePO₄, LiNiPO₄,LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄, LiFe_(a)Co_(b)PO₄,LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄, LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1,and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄, LiFe_(c)Ni_(D)Mn_(e)PO₄,LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1, and 0<e<1),LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1, 0<h<1, and0<i<1), and the like.

Alternatively, as the positive electrode active material, alithium-containing composite silicate represented by a general formulaLi(_(2-j))MSiO₄ (M is one or more of Fe(II), Mn(II), Co(II), and Ni(II);0≤j<2) can be used. Typical examples of the general formulaLi(_(2-j))MSiO₄ include Li(_(2-j))FeSiO₄, Li(_(2-j))NiSiO₄,Li(_(2-j))CoSiO₄, Li(_(2-j))MnSiO₄, Li(_(2-j))Fe_(k)Ni_(l)SiO₄,Li(_(2-j))Fe_(k)Co_(l)SiO₄, Li(_(2-j))Fe_(k)Mn_(l)SiO₄,Li(_(2-j))Ni_(k)Co_(l)SiO₄, Li(_(2-j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1,and 0<l<1), Li(_(2-j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li(_(2-j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li(_(2-j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1),Li(_(2-j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1), and the like.

In the case where carrier ions are alkaline-earth metal ions or alkalimetal ions other than lithium ions, the positive electrode activematerial layer 252 may contain, instead of lithium in the above lithiumcompound, lithium-containing composite phosphate, and lithium-containingcomposite silicate, an alkali metal (e.g., sodium or potassium) or analkaline-earth metal (e.g., calcium, strontium, barium, beryllium, ormagnesium).

The positive electrode active material layer 252 is not necessarilyformed in contact with the positive electrode current collector 251.Between the positive electrode current collector 251 and the positiveelectrode active material layer 252, any of the following functionallayers may be formed using a conductive material such as a metal: anadhesive layer for the purpose of improving adhesiveness between thepositive electrode current collector 251 and the positive electrodeactive material layer 252, a planarization layer for reducing unevennessof the surface of the positive electrode current collector 251, a heatradiation layer for radiating heat, and a stress relaxation layer forrelieving stress of the positive electrode current collector 251 or thepositive electrode active material layer 252.

FIG. 6B is a plan view of the positive electrode active material layer252. For the positive electrode active material layer 252, granularpositive electrode active materials 253 that can occlude and releasecarrier ions are used. An example is shown in which the positiveelectrode active material layer 252 includes graphenes 254 covering aplurality of granular positive electrode active materials 253 and atleast partly surrounding the plurality of granular positive electrodeactive materials 253. The different graphenes 254 cover surfaces of theplurality of granular positive electrode active materials 253. Thegranular positive electrode active materials 253 may be partly exposed.

The particle diameter of the granular positive electrode active material253 is preferably greater than or equal to 20 nm and less than or equalto 100 nm. Note that the particle diameter of the granular positiveelectrode active material 253 is preferably smaller because electronstransfer in the granular positive electrode active material 253.

Although sufficient characteristics can be obtained even when thesurface of the granular positive electrode active material 253 is notcovered with a graphite layer, it is preferable to use the granularpositive electrode active material 253 covered with a graphite layer, inwhich case hopping of carrier ions occurs between the granular positiveelectrode active materials 253, so that current flows.

FIG. 6C is a cross-sectional view of part of the positive electrodeactive material layer 252 in FIG. 6B. The positive electrode activematerial layer 252 includes the granular positive electrode activematerials 253 and the graphenes 254 covering a plurality of granularpositive electrode active materials 253. The graphene 254 has a linearshape when observed in the cross-sectional view. The plurality ofgranular positive electrode active materials 253 are at least partlysurrounded with one graphene 254 or a plurality of graphenes 254 orsandwiched between the plurality of graphenes 254. Note that thegraphene 254 has a bag-like shape and the plurality of granular positiveelectrode active materials 253 is surrounded by the graphene 254 in somecases. In addition, the positive electrode active materials are partlynot covered with the graphene 254 and exposed in some cases.

The desired thickness of the positive electrode active material layer252 is determined in the range of 20 μm to 100 μm. It is preferable toadjust the thickness of the positive electrode active material layer 252as appropriate so that cracks and separation do not occur.

Note that the positive electrode active material layer 252 may contain aknown conductive additive, for example, acetylene black particles havinga volume 0.1 to 10 times as large as that of the graphene or carbonparticles such as carbon nanofibers having a one-dimensional expansion.

Depending on a material of the positive electrode active material, thevolume is expanded by occlusion of ions serving as carriers. When such amaterial is used, the positive electrode active material layer getsvulnerable and is partly collapsed by charge and discharge, whichresults in lower reliability of a power storage device. However, evenwhen the volume of the positive electrode active material expands due tocharge and discharge, the graphene partly covers the periphery of thepositive electrode active material, which allows prevention ofdispersion of the positive electrode active material and the breakage ofthe positive electrode active material layer. That is to say, thegraphene has a function of maintaining the bond between the positiveelectrode active materials even when the volume of the positiveelectrode active materials fluctuates by charge and discharge.

The graphene 254 is in contact with the plurality of positive electrodeactive materials and serves also as a conductive additive. Further, thegraphene 254 has a function of holding the positive electrode activematerial capable of occluding and releasing carrier ions. Thus, a binderdoes not have to be mixed into the positive electrode active materiallayer. Accordingly, the amount of the positive electrode active materialin the positive electrode active material layer can be increased, whichallows an increase in discharge capacity of non-aqueous secondarybatteries.

Next, description is given of a method for forming the positiveelectrode active material layer 252.

First, slurry containing granular positive electrode active materialsand graphene oxide is formed. Next, the slurry is applied onto thepositive electrode current collector 251. Then, heating is performed ina reduced atmosphere for reduction treatment so that the positiveelectrode active materials are baked and oxygen included in the grapheneoxide is extracted to form graphene. Note that oxygen in the grapheneoxide is not entirely extracted and partly remains in the graphene.Through the above process, the positive electrode active material layer252 can be formed over the positive electrode current collector 251.Consequently, the positive electrode active material layer 252 hashigher conductivity.

Graphene oxide contains oxygen and thus is negatively charged in a polarsolvent. As a result of being negatively charged, graphene oxide isdispersed in the polar solvent. Therefore, the positive electrode activematerials contained in the slurry are not easily aggregated, so that theparticle diameter of the positive electrode active material can beprevented from increasing due to aggregation. Thus, the transfer ofelectrons in the positive electrode active materials is facilitated,resulting in an increase in conductivity of the positive electrodeactive material layer.

Next, a structure and a method for manufacturing a lithium secondarybattery are described with reference to FIGS. 7A and 7B. Here, across-sectional structure of the lithium ion secondary battery isdescribed below.

(Coin-Type Lithium Ion Battery)

FIG. 7A is an external view of a coin-type (single-layer flat type)lithium ion battery, and FIG. 7B is a cross-sectional view thereof.

In a coin-type lithium ion battery 300, a positive electrode can 301serving also as a positive electrode terminal and a negative electrodecan 302 serving also as a negative electrode terminal are insulated andsealed with a gasket 303 formed of polypropylene or the like. In amanner similar to that of the above, a positive electrode 304 includes apositive electrode current collector 305 and a positive electrode activematerial layer 306 which is provided to be in contact with the positiveelectrode current collector 305. On the other hand, a negative electrode307 includes a negative electrode current collector 308 and a negativeelectrode active material layer 309 which is provided to be in contactwith the negative electrode current collector 308. A separator 310 andan electrolyte (not illustrated) are included between the positiveelectrode active material layer 306 and the negative electrode activematerial layer 309.

As the negative electrode 307, the negative electrode described inEmbodiment 2 is used. As the positive electrode 304, the positiveelectrode 250 described in this embodiment can be used.

For the separator 310, an insulator such as cellulose (paper),polypropylene with pores, or polyethylene with pores can be used.

As an electrolyte of an electrolyte solution, a material which containscarrier ions is used. Typical examples of the electrolyte includelithium salts such as LiClO₄, LiAsF₆, LiBF₄, LiPF₆, and Li(C₂F₅SO₂)₂N.

In the case where carrier ions are alkali metal ions other than lithiumions or alkaline-earth metal ions, the electrolyte may contain, insteadof lithium in the lithium salts, an alkali metal (e.g., sodium orpotassium), an alkaline-earth metal (e.g., calcium, strontium, barium,beryllium, or magnesium).

As a solvent of the electrolyte solution, a material in which thecarrier ions can transfer is used. As the solvent of the electrolytesolution, an aprotic organic solvent is preferably used. Typicalexamples of aprotic organic solvents include ethylene carbonate (EC),propylene carbonate, dimethyl carbonate, diethyl carbonate (DEC),γ-butyrolactone, acetonitrile, dimethoxyethane, and tetrahydrofuran, andone or more of these materials can be used. With the use of a gelledhigh-molecular material as the solvent of the electrolyte solution,safety against liquid leakage and the like is improved. Furthermore, alithium ion battery can be thinner and more lightweight. Typicalexamples of gelled high-molecular materials include a silicone gel, anacrylic gel, an acrylonitrile gel, polyethylene oxide, polypropyleneoxide, and a fluorine-based polymer. Alternatively, the use of one ormore of ionic liquids (room temperature molten salts) which are lesslikely to burn and volatilize as the solvent of the electrolyte solutioncan prevent the lithium ion battery from exploding or catching fire evenwhen the secondary battery internally shorts out or the internaltemperature increases due to overcharging or the like.

Instead of the electrolyte solution, a solid electrolyte including asulfide-based inorganic material, an oxide-based inorganic material, orthe like, or a solid electrolyte including a polyethylene oxide(PEO)-based high-molecular material or the like can be used. In the caseof using the solid electrolyte, a separator is not necessary. Further,the battery can be entirely solidified; therefore, there is nopossibility of liquid leakage and thus the safety of the battery isdramatically increased.

For the positive electrode can 301 and the negative electrode can 302, acorrosion-resistant metal such as nickel, aluminum, or titanium, analloy of such a metal, or an alloy of such a metal and another metal(e.g., stainless steel or the like) can be used. Alternatively, thepositive electrode can 301 and the negative electrode can 302 arepreferably covered with nickel, aluminum, or the like in order toprevent corrosion by the electrolyte solution. The positive electrodecan 301 and the negative electrode can 302 are electrically connected tothe positive electrode 304 and the negative electrode 307, respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolyte solution. Then, asillustrated in FIG. 7B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type lithium ionbattery 300 is manufactured.

(Laminated Lithium Ion Battery)

Next, an example of a laminated lithium ion battery is described withreference to FIG. 8.

In a laminated lithium ion battery 400 illustrated in FIG. 8, a positiveelectrode 403 including a positive electrode current collector 401 and apositive electrode active material layer 402, a separator 407, and anegative electrode 406 including a negative electrode current collector404 and a negative electrode active material layer 405 are stacked andsealed in an exterior body 409, and then an electrolyte solution 408 isinjected into the exterior body 409. Although FIG. 8 illustrates thelaminated lithium ion battery 400 with a structure in which onesheet-like positive electrode 403 and one sheet-like negative electrode406 are stacked, to increase the capacity of the battery, the stack ispreferably wound or a plurality of positive electrodes and negativeelectrodes are stacked and then laminated. Particularly in the case ofthe laminated lithium ion battery, the battery has flexibility and thusis suitable for applications which require flexibility.

In the laminated lithium ion battery 400 illustrated in FIG. 8, thepositive electrode current collector 401 and the negative electrodecurrent collector 404 serve as terminals for an electrical contact withthe outside. For this reason, the positive electrode current collector401 and the negative electrode current collector 404 are arranged sothat part of the positive electrode current collector 401 and part ofthe negative electrode current collector 404 are exposed outside theexterior body 409.

As the exterior body 409 in the laminated lithium ion battery 400, forexample, a laminate film having a three-layer structure in which ahighly flexible metal thin film of aluminum, stainless steel, copper,nickel, or the like is provided over the inner surface of a film formedof a material such as polyethylene, polypropylene, polycarbonate,ionomer, or polyamide, and an insulating synthetic resin film of apolyamide-based resin, a polyester-based resin, or the like is providedas the outer surface of the exterior body over the metal thin film canbe used. With such a three-layer structure, permeation of anelectrolytic solution and a gas can be blocked and an insulatingproperty and resistance to the electrolytic solution can be provided.

(Cylindrical Lithium Ion Battery)

Next, an example of a cylindrical lithium ion battery is described withreference to FIGS. 9A and 9B. As illustrated in FIG. 9A, a cylindricallithium ion battery 500 includes a positive electrode cap (battery lid)501 on its top surface and a battery can (exterior can) 502 on its sidesurface and bottom surface. The positive electrode cap 501 and thebattery can 502 are insulated from each other by a gasket 510(insulating packing).

FIG. 9B is a diagram schematically illustrating a cross section of thecylindrical lithium ion battery. In the battery can 502 with a hollowcylindrical shape, a battery element is provided in which a strip-likepositive electrode 504 and a strip-like negative electrode 506 are woundwith a separator 505 provided therebetween. Although not illustrated,the battery element is wound around a center pin as a center. One end ofthe battery can 502 is close and the other end thereof is open. For thebattery can 502, a corrosion-resistant metal such as nickel, aluminum,or titanium, an alloy of such a metal, or an alloy of such a metal andanother metal (e.g., stainless steel or the like) can be used.Alternatively, the battery can 502 is preferably covered with nickel,aluminum, or the like in order to prevent corrosion by the electrolytesolution. Inside the battery can 502, the battery element in which thepositive electrode, the negative electrode, and the separator are woundis interposed between a pair of insulating plates 508 and 509 which faceeach other. Further, an electrolyte solution (not illustrated) isinjected inside the battery can 502 in which the battery element isprovided. An electrolyte solution which is similar to that of thecoin-type lithium ion battery or the laminated lithium ion battery canbe used.

Although the positive electrode 504 and the negative electrode 506 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type lithium ion battery, the differencelies in that, since the positive electrode and the negative electrode ofthe cylindrical lithium ion battery are wound, active materials areformed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 503 is connectedto the positive electrode 504, and a negative electrode terminal(negative electrode current collecting lead) 507 is connected to thenegative electrode 506. A metal material such as aluminum can be usedfor both the positive electrode terminal 503 and the negative electrodeterminal 507. The positive electrode terminal 503 is resistance-weldedto a safety valve mechanism 512, and the negative electrode terminal 507is resistance-welded to the bottom of the battery can 502. The safetyvalve mechanism 512 is electrically connected to the positive electrodecap 501 through a positive temperature coefficient (PTC) element 511.The safety valve mechanism 512 cuts off electrical connection betweenthe positive electrode cap 501 and the positive electrode 504 when theinternal pressure of the battery increases and exceeds a predeterminedthreshold value. The PTC element 511 is a heat sensitive resistor whoseresistance increases as temperature rises, and controls the amount ofcurrent by increase in resistance to prevent unusual heat generation.Barium titanate (BaTiO₃)-based semiconductor ceramic or the like can beused for the PTC element.

Note that in this embodiment, the coin-type lithium ion battery, thelaminated lithium ion battery, and the cylindrical lithium ion batteryare given as examples of the lithium ion battery; however, any oflithium ion batteries with various shapes, such as a sealing-typelithium ion battery and a square-type lithium ion battery, can be used.Further, a structure in which a plurality of positive electrodes, aplurality of negative electrodes, and a plurality of separators arestacked or wound may be employed.

The electrode for a power storage device which is one embodiment of thepresent invention is used as the negative electrode in each of thelithium ion battery 300, the lithium ion battery 400, and the lithiumion battery 500 described in this embodiment. Thus, the lithium ionbattery 300, the lithium ion battery 400, and the lithium ion battery500 can have favorable cycle performance. For example, after 500 cyclesof charge and discharge, the capacity of the power storage device ispreferably higher than or equal to 60% of the initial capacity. Inaddition, irreversible capacity generated in initial charge anddischarge can be reduced; moreover, a lithium ion battery with favorablehigh temperature characteristics can be provided.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 4

In this embodiment, a lithium ion capacitor is described as a powerstorage device.

The lithium ion capacitor is a hybrid capacitor which combines apositive electrode of an electrical double layer capacitor (EDLC) and anegative electrode of a lithium ion battery using a carbon material, andalso an asymmetric capacitor in which the principles of power storageare different between the positive electrode and the negative electrode.The positive electrode forms an electrical double layer and enablescharge and discharge by a physical action, whereas the negativeelectrode enables charge and discharge by a chemical action of lithium.With the use of a negative electrode in which lithium is occluded inadvance as the carbon material or the like that is a negative electrodeactive material, the lithium ion capacitor can have energy densitydramatically higher than that of a conventional electrical double layercapacitor including a negative electrode using active carbon.

In the lithium ion capacitor, instead of the positive electrode activematerial layer in the lithium ion battery described in Embodiment 3, amaterial capable of reversibly having at least one of lithium ions andanions is used. Examples of such a material include active carbon, aconductive high molecule, and a polyacene-based organic semiconductor(PAS).

The lithium ion capacitor has high efficiency of charge and discharge,capability of rapidly performing charge and discharge, and a long lifeeven when it is repeatedly used.

As the negative electrode of such a lithium ion capacitor, the negativeelectrode of a power storage device which is described in Embodiment 2is used. Thus, irreversible capacity generated in initial charge anddischarge is reduced, so that a power storage device having improvedcycle performance can be manufactured. Furthermore, a power storagedevice having excellent high temperature characteristics can bemanufactured.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 5

A power storage device of one embodiment of the present invention can beused as a power supply of various electrical appliances which are drivenby electric power.

Specific examples of electrical appliances each using the power storagedevice of one embodiment of the present invention are as follows:display devices of televisions, monitors, and the like, lightingdevices, desktop personal computers and laptop personal computers, wordprocessors, image reproduction devices which reproduce still images andmoving images stored in recording media such as digital versatile discs(DVDs), portable CD players, portable radios, tape recorders, headphonestereos, stereos, table clocks, wall clocks, cordless phone handsets,transceivers, mobile phones, car phones, portable game machines,calculators, portable information terminals, electronic notebooks,e-book readers, electronic translators, audio input devices, videocameras, digital still cameras, toys, electric shavers, high-frequencyheating appliances such as microwave ovens, electric rice cookers,electric washing machines, electric vacuum cleaners, water heaters,electric fans, hair dryers, air-conditioning systems such as airconditioners, humidifiers, and dehumidifiers, dishwashers, dish dryers,clothes dryers, futon dryers, electric refrigerators, electric freezers,electric refrigerator-freezers, freezers for preserving DNA,flashlights, electric power tools such as chain saws, smoke detectors,and medical equipment such as dialyzers. The examples also includeindustrial equipment such as guide lights, traffic lights, beltconveyors, elevators, escalators, industrial robots, power storagesystems, and power storage devices for leveling the amount of powersupply and smart grid. In addition, moving objects driven by an electricmotor using power from a power storage device are also included in thecategory of electrical appliances. Examples of the moving objects areelectric vehicles (EV), hybrid electric vehicles (HEV) which includeboth an internal-combustion engine and a motor, plug-in hybrid electricvehicles (PHEV), tracked vehicles in which caterpillar tracks aresubstituted for wheels of these vehicles, motorized bicycles includingmotor-assisted bicycles, motorcycles, electric wheelchairs, golf carts,boats, ships, submarines, helicopters, aircrafts, rockets, artificialsatellites, space probes, planetary probes, and spacecrafts.

In the above electrical appliances, the power storage device of oneembodiment of the present invention can be used as a main power sourcefor supplying enough power for almost the whole power consumption.Alternatively, in the above electrical appliances, the power storagedevice of one embodiment of the present invention can be used as anuninterruptible power source which can supply power to the electricalappliances when the supply of power from the main power source or acommercial power source is stopped. Still alternatively, in the aboveelectrical appliances, the power storage device of one embodiment of thepresent invention can be used as an auxiliary power source for supplyingpower to the electrical appliances at the same time as the power supplyfrom the main power source or a commercial power source.

FIG. 10 illustrates specific structures of the electrical appliances. InFIG. 10, a display device 600 is an example of an electrical applianceusing a power storage device 604 of one embodiment of the presentinvention. Specifically, the display device 600 corresponds to a displaydevice for TV broadcast reception and includes a housing 601, a displayportion 602, speaker portions 603, the power storage device 604, and thelike. The power storage device 604 of one embodiment of the presentinvention is provided in the housing 601. The display device 600 canreceive power from a commercial power source. Alternatively, the displaydevice 600 can use power stored in the power storage device 604. Thus,the display device 600 can be operated with the use of the power storagedevice 604 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoretic displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 602.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like in addition to TV broadcast reception.

In FIG. 10, an installation lighting device 610 is an example of anelectrical appliance using a power storage device 613 of one embodimentof the present invention. Specifically, the installation lighting device610 includes a housing 611, a light source 612, the power storage device613, and the like. Although FIG. 10 illustrates the case where the powerstorage device 613 is provided in a ceiling 614 on which the housing 611and the light source 612 are installed, the power storage device 613 maybe provided in the housing 611. The installation lighting device 610 canreceive power from a commercial power source. Alternatively, theinstallation lighting device 610 can use power stored in the powerstorage device 613. Thus, the installation lighting device 610 can beoperated with the use of the power storage device 613 of one embodimentof the present invention as an uninterruptible power source even whenpower cannot be supplied from a commercial power source due to powerfailure or the like.

Note that although the installation lighting device 610 provided in theceiling 614 is illustrated in FIG. 10 as an example, the power storagedevice of one embodiment of the present invention can be used as aninstallation lighting device provided in, for example, a wall 615, afloor 616, a window 617, or the like other than the ceiling 614.Alternatively, the power storage device can be used in a tabletoplighting device or the like.

As the light source 612, an artificial light source which emits lightartificially by using power can be used. Specifically, an incandescentlamp, a discharge lamp such as a fluorescent lamp, and a light-emittingelement such as an LED or an organic EL element are given as examples ofthe artificial light source.

In FIG. 10, an air conditioner including an indoor unit 620 and anoutdoor unit 624 is an example of an electrical appliance using a powerstorage device 623 of one embodiment of the present invention.Specifically, the indoor unit 620 includes a housing 621, an air outlet622, the power storage device 623, and the like. Although FIG. 10illustrates the case where the power storage device 623 is provided inthe indoor unit 620, the power storage device 623 may be provided in theoutdoor unit 624. Alternatively, the power storage device 623 may beprovided in both the indoor unit 620 and the outdoor unit 624. The airconditioner can receive power from a commercial power source.Alternatively, the air conditioner can use power stored in the powerstorage device 623. Particularly in the case where the power storagedevices 623 are provided in both the indoor unit 620 and the outdoorunit 624, the air conditioner can be operated with the use of the powerstorage device 623 of one embodiment of the present invention as anuninterruptible power source even when power cannot be supplied from acommercial power source due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 10 as an example, thepower storage device of one embodiment of the present invention can beused in an air conditioner in which the functions of an indoor unit andan outdoor unit are integrated in one housing.

In FIG. 10, an electric refrigerator-freezer 630 is an example of anelectrical appliance using a power storage device 634 of one embodimentof the present invention. Specifically, the electricrefrigerator-freezer 630 includes a housing 631, a door for arefrigerator 632, a door for a freezer 633, the power storage device634, and the like. The power storage device 634 is provided inside thehousing 631 in FIG. 10. The electric refrigerator-freezer 630 canreceive power from a commercial power source. Alternatively, theelectric refrigerator-freezer 630 can use power stored in the powerstorage device 634. Thus, the electric refrigerator-freezer 630 can beoperated with the use of the power storage device 634 of one embodimentof the present invention as an uninterruptible power source even whenpower cannot be supplied from a commercial power source due to powerfailure or the like.

Note that among the electrical appliances described above, ahigh-frequency heating apparatus such as a microwave oven and anelectrical appliance such as an electric rice cooker require high powerin a short time. The tripping of a circuit breaker of a commercial powersource in use of electrical appliances can be prevented by using thepower storage device of one embodiment of the present invention as anauxiliary power source for supplying power which cannot be suppliedenough by a commercial power source.

In addition, in a time period when electrical appliances are not used,particularly when the proportion of the amount of power which isactually used to the total amount of power which can be supplied from acommercial power source (such a proportion referred to as a usage rateof power) is low, power can be stored in the power storage device,whereby the usage rate of power can be reduced in a time period when theelectrical appliances are used. For example, in the case of the electricrefrigerator-freezer 630, power can be stored in the power storagedevice 634 in nighttime when the temperature is low and the door for arefrigerator 632 and the door for a freezer 633 are not often opened andclosed. On the other hand, in daytime when the temperature is high andthe door for a refrigerator 632 and the door for a freezer 633 arefrequently opened and closed, the power storage device 634 is used as anauxiliary power source; thus, the usage rate of power in daytime can bereduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Embodiment 6

Next, a portable information terminal which is an example of anelectrical appliance is described with reference to FIGS. 11A to 11C.

FIGS. 11A and 11B illustrate a tablet terminal 650 that can be folded.FIG. 11A illustrates the tablet terminal 650 in the state of beingunfolded. The tablet terminal 650 includes a housing 651, a displayportion 652 a, a display portion 652 b, a switch 653 for switchingdisplay modes, a power switch 654, a switch 655 for switching topower-saving-mode, and an operation switch 656.

Part of the display portion 652 a can be a touch panel region 657 a anddata can be input when a displayed operation key 658 is touched. Notethat FIG. 1i A illustrates, as an example, that half of the area of thedisplay portion 652 a has only a display function and the other half ofthe area has a touch panel function. However, the structure of thedisplay portion 652 a is not limited to this, and all the area of thedisplay portion 652 a may have a touch panel function. For example, allthe area of the display portion 652 a can display keyboard buttons andserve as a touch panel while the display portion 652 b can be used as adisplay screen.

Like the display portion 652 a, part of the display portion 652 b can bea touch panel region 657 b. When a finger, a stylus, or the like touchesthe place where a button 659 for switching to keyboard display isdisplayed in the touch panel, keyboard buttons can be displayed on thedisplay portion 652 b.

Touch input can be performed on the touch panel regions 657 a and 657 bat the same time.

The switch 653 for switching display modes can switch the displaybetween portrait mode, landscape mode, and the like, and betweenmonochrome display and color display, for example. With the switch 655for switching to power-saving mode, the luminance of display can beoptimized depending on the amount of external light at the time when thetablet terminal is in use, which is detected with an optical sensorincorporated in the tablet terminal. The tablet terminal may includeanother detection device such as a sensor for detecting orientation(e.g., a gyroscope or an acceleration sensor) in addition to the opticalsensor.

Although the display area of the display portion 652 a is the same asthat of the display portion 652 b in FIG. 11A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 652 a may be different from that of the displayportion 652 b, and further, the display quality of the display portion652 a may be different from that of the display portion 652 b. Forexample, one of them may be a display panel that can displayhigher-definition images than the other.

FIG. 11B illustrates the tablet terminal 650 in the state of beingclosed. The tablet terminal 650 includes the housing 651, a solar cell660, a charge and discharge control circuit 670, a battery 671, and aDCDC converter 672. Note that FIG. 11B illustrates an example in whichthe charge and discharge control circuit 670 includes the battery 671and the DCDC converter 672, and the battery 671 includes the powerstorage device described in any of the above embodiments.

Since the tablet terminal 650 can be folded, the housing 651 can beclosed when the tablet terminal 650 is not in use. Thus, the displayportions 652 a and 652 b can be protected, thereby providing the tabletterminal 650 with excellent endurance and excellent reliability forlong-term use.

The tablet terminal illustrated in FIGS. 11A and 11B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 660, which is attached on the surface of the tabletterminal, supplies power to the touch panel, the display portion, avideo signal processor, and the like. Note that the solar cell 660 ispreferably provided on one or two surfaces of the housing 651, in whichcase the battery 671 can be charged efficiently. The use of the powerstorage device of one embodiment of the present invention as the battery671 has advantages such as a reduction is size.

The structure and operation of the charge and discharge control circuit670 illustrated in FIG. 11B are described with reference to a blockdiagram in FIG. 11C. The solar cell 660, the battery 671, the DCDCconverter 672, a converter 673, switches SW1 to SW3, and the displayportion 652 are illustrated in FIG. 11C, and the battery 671, the DCDCconverter 672, the converter 673, and the switches SW1 to SW3 correspondto the charge and discharge control circuit 670 illustrated in FIG. 11B.

First, an example of the operation in the case where power is generatedby the solar cell 660 using external light is described. The voltage ofpower generated by the solar cell 660 is raised or lowered by the DCDCconverter 672 so that the power has a voltage for charging the battery671. Then, when the power from the solar cell 660 is used for theoperation of the display portion 652, the switch SW1 is turned on andthe voltage of the power is raised or lowered by the converter 673 so asto be a voltage needed for the display portion 652. In addition, whendisplay on the display portion 652 is not performed, the switch SW1 maybe turned off and the switch SW2 may be turned on so that the battery671 is charged.

Here, the solar cell 660 is described as an example of a powergeneration means; however, there is no particular limitation on thepower generation means, and the battery 671 may be charged with anotherpower generation means such as a piezoelectric element or athermoelectric conversion element (Peltier element). For example, thebattery 671 may be charged with a non-contact power transmission modulethat transmits and receives power wirelessly (without contact) to chargethe battery or with a combination of other charging means.

It is needless to say that one embodiment of the present invention isnot limited to the electrical appliance illustrated in FIGS. 11A to 11Cas long as the electrical appliance is equipped with the power storagedevice described in any of the above embodiments.

Embodiment 7

Further, an example of the moving object which is an example of theelectrical appliance is described with reference to FIGS. 12A and 12B.

Any of the power storage device described in any of the aboveembodiments can be used as a control battery. The control battery can beexternally charged by electric power supply using a plug-in technique orcontactless power feeding. Note that in the case where the moving objectis an electric railway vehicle, the electric railway vehicle can becharged by electric power supply from an overhead cable or a conductorrail.

FIGS. 12A and 12B illustrate an example of an electric vehicle. Anelectric vehicle 680 is equipped with a battery 681. The output of thepower of the battery 681 is adjusted by a control circuit 682 and thepower is supplied to a driving device 683. The control circuit 682 iscontrolled by a processing unit 684 including a ROM, a RAM, a CPU, orthe like which is not illustrated.

The driving device 683 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 684 outputs a control signal to the control circuit 682 based oninput data such as data on operation (e.g., acceleration, deceleration,or stop) by a driver of the electric vehicle 680 or data on driving theelectric vehicle 680 (e.g., data on an upgrade or a downgrade, or dataon a load on a driving wheel). The control circuit 682 adjusts theelectric energy supplied from the battery 681 in response to the controlsignal of the processing unit 684 to control the output of the drivingdevice 683. In the case where the AC motor is mounted, although notillustrated, an inverter which converts direct current into alternatecurrent is also incorporated.

The battery 681 can be charged by external electric power supply using aplug-in technique. For example, the battery 681 is charged through apower plug from a commercial power source. The battery 681 can becharged by converting external power into DC constant voltage having apredetermined voltage level through a converter such as an ACDCconverter. When the power storage device of one embodiment of thepresent invention is provided as the battery 681, capacity of thebattery 681 can be increased and improved convenience can be realized.When the battery 681 itself can be made compact and lightweight withimproved characteristics of the battery 681, the vehicle can be madelightweight, leading to an increase in fuel efficiency.

Note that it is needless to say that one embodiment of the presentinvention is not limited to the electrical appliance described above aslong as the power storage device of one embodiment of the presentinvention is included.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

Example

In this example, a negative electrode of a power storage device, whichincludes a coating film formed of a silicon oxide, and a power storagedevice using the negative electrode, which are actually manufactured,will be described.

Reference Example

As a reference example, description will be made on an example of anelectrode in which a silicon oxide film is formed over a layer servingas an active material.

In the reference example, a 100-μm-thick titanium sheet TR270C producedby JX Nippon Mining & Metals Corporation was used as a currentcollector, and a 200-nm-thick silicon film was formed over the currentcollector by a thermal CVD method. Moreover, SiO₂ powder was pelletizedand deposited by electron beam heating, whereby a 100-nm-thick siliconoxide film was formed over the silicon film. Thus, an electrode (alsoreferred to as an electrode A) was formed. An electrode (also referredto as an electrode B) was formed as a comparative example by formingonly a 200-nm-thick silicon film over a current collector formed withthe same material as the current collector in the electrode A.

A three-electrode cell (also referred to as a cell A) in which theelectrode A is used as a working electrode and a three-electrode cell(also referred to as a cell B) in which the electrode B is used as aworking electrode were fabricated. In this case, lithium was used for areference electrode and a counter electrode in a three-electrodeelectrochemical measurement cell. In addition, an electrolyte solutionwas formed in such a way that lithium hexafluorophosphate (LiPF₆) wasdissolved at a concentration of 1 mol/L in a solution in which ethylenecarbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratioof 1:1.

The cell A and the cell B are subjected to cyclic voltammetry (CV)measurement. The potential range in the CV measurement was 0 V to 2 V(vs. Li/Li), and scanning was performed only in the negative direction.The sweep rate in an electric field was set to 0.1 mV/sec. FIG. 13 isthe cyclic voltammogram showing the CV measurement results.

FIG. 13 indicates that even in the case where the silicon film iscovered with the silicon oxide film, lithium ions are inserted into thesilicon film serving as an active material. The silicon oxide film has afunction of allowing passage of lithium ions, and lithium ions reactwith the silicon film.

(Example of Power Storage Device)

As an example, a negative electrode of a power storage device and apower storage device using the negative electrode, which are actuallymanufactured, will be described.

In this example, graphite particles provided with a silicon oxide filmwere formed by a sol-gel method. For the graphite particles, graphiteproduced by JFE Chemical Corporation was used. First, silicon ethoxide,ethyl acetoacetate, and toluene were mixed and stirred to form aSi(OEt)₄ toluene solution, as described in Embodiment 1. At this time,the amount of silicon ethoxide was determined so that the proportion ofsilicon oxide formed later in graphite was 1 wt % (weight percent). Thecompounding ratio of this solution was as follows: Si(OEt)₄ was3.14×10⁻⁴ mol; ethyl acetoacetate, 6.28×10⁻⁴ mol; and toluene, 2 ml.Next, graphite was added to the Si(OEt)₄ toluene solution and themixture was stirred in a dry room. Then, the solution was held at 70° C.in a humid environment for 3 hours so that Si(OEt)₄ in the Si(OEt)₄toluene solution to which graphite was added was hydrolyzed andcondensed. In other words, Si(OEt)₄ in the solution was made to reactwith water in the air to gradually cause hydrolysis reaction, andSi(OEt)₄ was condensed by dehydration reaction which sequentiallyoccurred. In such a manner, silicon which is a gel was attached onto asurface of graphite particles to form a net-like structure formed ofC—O—Si bonds. Then, baking was performed in a nitrogen atmosphere at500° C. for 3 hours, thereby forming an electrode material containingthe graphite particles covered with a coating film formed of a siliconoxide. In addition, slurry formed by mixing the electrode material,acetylene black, and PVDF was applied onto a current collector anddried; thus, an electrode (also referred to as Electrode 1) was formed.At this time, the weight ratio of PVDF to graphite was 10 wt % (weightpercent).

FIG. 14 is an observation image of Electrode 1 taken with a scanningelectron microscope (SEM). FIG. 14 shows that a plurality of particles2010 is formed. The plurality of particles has an average diameter ofapproximately 9 μm.

In addition, observation with a scanning transmission electronmicroscope (STEM) and energy dispersive X-ray spectroscopy (EDX) wereperformed on Electrode 1. FIGS. 15A and 15B show the results of theobservation and analysis. The results of EDX in FIGS. 15A and 15B areobtained by line scanning.

In the STEM image in FIG. 15A, a relatively dark gray portioncorresponds to the particle 2010. The plurality of particles 2010 can beobserved.

A relatively light gray region 2011 exists between the plurality ofparticles 2010. From the result of EDX, silicon is detected in the lineA-B across part of the particle 2010 region and part of the light grayregion 2011. This indicates that the light gray region 2011 correspondsto the silicon oxide film. On the other hand, from the STEM image inFIG. 15B and the result of EDX in the line C-D, the silicon oxide filmis not observed. Therefore, it is found that the silicon oxide film isnot formed on the entire surface of the particles 2010, but formed onpart of the surface of the particles 2010.

(CV Measurement)

Next, whether the silicon oxide film of Electrode 1 has an effect ofsuppressing decomposition of the electrolyte solution or not wasexamined by CV measurement.

For the CV measurement, a three-electrode cell was used, an electrode Xwas used as a working electrode, lithium was used for a referenceelectrode and a counter electrode, and a solution obtained by dissolving1 mol/L of lithium hexafluorophosphate (LiPF₆) in a mixed solution ofethylene carbonate (EC) and diethyl carbonate (DEC) (volume ratio 1:1)was used as an electrolyte solution. The measurement was performed at ascan rate (the sweep rate in an electric field) of 4 μV/sec in thepotential range of 0.01 V to 1 V (vs. Li/Li⁺).

FIGS. 16A and 16B show the results of CV measurement of one cycle. FIG.16A shows the results of measurement in the scan range of 0.01 V to 1 V.FIG. 16B is a graph focused on potentials around 0.4 V to 1V.

In FIG. 16B, a peak 2002 appears in the range of 0.7 V to 1 V. Thisindicates decomposition of the electrolyte solution.

For the comparison, Electrode 2 in which a silicon oxide film is notprovided on a surface of graphite particles which are the same as theabove-described graphite particles was made, and CV measurement wasperformed for two cycles under the same conditions. FIGS. 17A and 17Bshow the results obtained by comparing a cell using Electrode 1 with acell using Electrode 2. FIG. 17A is a cyclic voltammogram, and FIG. 17Bis a graph showing the capacity of decomposition of the electrolytesolution, which is calculated on the basis of the results in FIG. 17A.

As shown in FIGS. 17A and 17B, the peak 2002 that appears in the rangeof 0.7 V to 1 V in the cell using Electrode 2 is higher than that in thecell using Electrode 1. Therefore, decomposition of the electrolytesolution can be suppressed by providing a silicon oxide film.

(Cycle Performance Evaluation)

A negative electrode X and a negative electrode Y were formed. In thenegative electrode X, graphite particles provided with a silicon oxidefilm, which is formed by the above-mentioned sol-gel method, are used asnegative electrode active materials. In the negative electrode Y,graphite particles provided with a silicon oxide film, which is formedby the above-mentioned polysilazane method, are used as negativeelectrode active materials. A cell using LiFePO₄ as a positive electrodeand the negative electrode X and a cell using LiFePO₄ as a positiveelectrode and the negative electrode Y were formed, and the cycleperformance of the cells were compared with each other.

The negative electrode X using a sol-gel method was made in a mannersimilar to that of Electrode 1.

In the formation of the negative electrode Y using a polysilazanemethod, 5 g of graphite produced by JFE Chemical Corporation and 2.5 mlof toluene were mixed in a dry room; 1.3 mg of a xylene solutioncontaining 20 wt % perhydropolysilazane was added thereto; and themixture was further mixed in the dry room. The mixture was kept in theair for 30 minutes, subjected to heat treatment with a hot plate in theair at 150° C. for one hour, and dried with a glass tube oven at 170° C.for 0 hours. Thus, an electrode material including graphite particlesprovided with a silicon oxide film was formed. In addition, slurryformed by mixing the electrode material, acetylene black, and PVDF wasapplied onto a current collector of copper with a thickness of 18 μm anddried. At this time, the weight ratio of PVDF to graphite was 10 wt %(weight percent).

Note that the negative electrode X and the negative electrode Y weremade such that the weight ratio of the silicon oxide film to thegraphite particles was 1 wt % (weight percent).

The performance was measured using coin cells. An electrolyte solutionformed in such a manner that lithium hexafluorophosphate (LiPF₆) wasdissolved at a concentration of 1 mol/L in a solution in which ethylenecarbonate (EC) and diethyl carbonate (DEC) were mixed at a volume ratioof 1:1 was used. As the separator, polypropylene (PP) was used. Chargeand discharge were performed at a rate of 1 C (it takes 1 hour forcharging), voltages ranging from 2 V to 4 V, and an environmenttemperature of 60° C. Under such conditions, the measurement wasperformed.

The cycle performance evaluation was performed on a secondary batteryusing the negative electrode X, a secondary battery using the negativeelectrode Y, and a secondary battery using a negative electrode Z. Thenegative electrode Z was made for comparison, in which graphiteparticles which are not provided with the coating film are used asnegative electrode active materials.

FIG. 18 shows the results of the cycle performance evaluation. Thehorizontal axis represents the number of cycles (times) and the verticalaxis represents capacity retention (%) of the secondary batteries. Thenumber of measured samples of each of the secondary battery using thenegative electrode X, the secondary battery using the negative electrodeY, and the secondary battery using the negative electrode Z was two(n=2).

FIG. 18 shows that as the number of cycles increases, the dischargecapacities of the secondary batteries using the negative electrode X andsecondary batteries using the negative electrode Y are less likely todecrease than those of the secondary batteries using the negativeelectrode Z at 60° C. For example, after 500 cycles of charge anddischarge, the capacity of the secondary battery using the negativeelectrode X is higher than or equal to 60% of the initial capacity.Thus, decomposition reaction of the electrolyte solution, which speedsup at high temperature, is suppressed and a decrease in capacity incharge and discharge at high temperature is suppressed, so that theoperating temperature range of a power storage device can be extended.

The decrease in discharge capacity of the secondary battery using thenegative electrode X is smaller than the decrease in discharge capacityof the secondary battery using the negative electrode Y. A net-likestructure is less likely formed on the negative electrode activematerial produced by a polysilazane method as compared with on thenegative electrode active material produced by a sol-gel method. This isbecause Si(OH) is needed for forming the net-like structure, and theamount of Si(OH) generated by the method of producing the electrodematerial using a polysilazane method is smaller than the amount ofSi(OH) generated by the method of producing the electrode material usinga sol-gel method. Therefore, formation of the net-like structure leadsto improvement in cycle performance and reliability of a power storagedevice and.

This application is based on Japanese Patent Application serial No.2012-224581 fled with Japan Patent Office on Oct. 9, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A material for an electrode of a power storagedevice comprising: a granular active material comprising a carbon atom;and a film having a structure over the granular active material, whereinthe structure comprises a plurality of bonds between the carbon atom andone of a silicon atom and a metal atom through an oxygen atom, andwherein the film partly covers a surface of the granular active materialso that the surface of the granular active material has a first regionwhich is covered with the film, and a second region which is not coveredwith the film.
 2. The material for an electrode of a power storagedevice according to claim 1, wherein the granular active materialcomprises a graphite particle.
 3. The material for an electrode of apower storage device according to claim 1, wherein the metal atom is oneof a niobium atom, a titanium atom, a vanadium atom, a tantalum atom, atungsten atom, a zirconium atom, a molybdenum atom, a hafnium atom, achromium atom, and an aluminum atom.
 4. The material for an electrode ofa power storage device according to claim 1, further comprising aplurality of oxide layers over the film, wherein each of the pluralityof oxide layers comprises a bond of the one of the silicon atom and themetal atom, and an oxygen atom.
 5. A power storage device comprising anegative electrode, the negative electrode comprising the material foran electrode of a power storage device according to claim
 1. 6. Anelectrical appliance comprising the power storage device according toclaim
 5. 7. A material for an electrode of a power storage devicecomprising: a granular active material comprising a carbon atom; and afilm having a structure over the granular active material, wherein thestructure comprises a plurality of bonds between the carbon atom and asilicon atom through an oxygen atom, and wherein the film partly coversa surface of the granular active material so that the surface of thegranular active material has a first region which is covered with thefilm, and a second region which is not covered with the film.
 8. Thematerial for an electrode of a power storage device according to claim7, wherein the granular active material comprises a graphite particle.9. The material for an electrode of a power storage device according toclaim 7, further comprising a plurality of oxide layers over the film,wherein each of the plurality of oxide layers comprises a bond of thesilicon atom and an oxygen atom.
 10. A power storage device comprising anegative electrode, the negative electrode comprising the material foran electrode of a power storage device according to claim
 7. 11. Anelectrical appliance comprising the power storage device according toclaim
 10. 12. A material for an electrode of a power storage devicecomprising: a granular active material comprising a carbon atom; and afilm having a structure over the granular active material, wherein thestructure comprises a plurality of bonds between the carbon atom and oneof a silicon atom and a metal atom through an oxygen atom, wherein thegranular active material comprises a plurality of graphene layers,wherein the film partly covers a surface of the granular active materialso that the surface of the granular active material has a first regionwhich is covered with the film, and a second region which is not coveredwith the film, and wherein the structure exists at end portions of partof the plurality of graphene layers.
 13. The material for an electrodeof a power storage device according to claim 12, wherein the metal atomis one of a niobium atom, a titanium atom, a vanadium atom, a tantalumatom, a tungsten atom, a zirconium atom, a molybdenum atom, a hafniumatom, a chromium atom, and an aluminum atom.
 14. The material for anelectrode of a power storage device according to claim 12, furthercomprising a plurality of oxide layers over the film, wherein each ofthe plurality of oxide layers comprises a bond of the one of the siliconatom and the metal atom, and an oxygen atom.
 15. A power storage devicecomprising a negative electrode, the negative electrode comprising thematerial for an electrode of a power storage device according to claim12.
 16. An electrical appliance comprising the power storage deviceaccording to claim 15.